Hydroxyapatite-targeting poly (ethylene glycol) and related polymers

ABSTRACT

Isolatable, hydroxyapatite-targeting polymeric structures, and biologically active conjugates thereof, are provided. The polymeric structure includes a linear or branched water-soluble and non-peptidic polymer backbone, such as a PEG backbone, having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety, such as a bisphosphonate, and a second terminus covalently bonded to a chemically reactive group, wherein said chemically reactive group is protected or unprotected. Methods of preparing and using hydroxyapatite-targeting polymeric structures, and biologically active conjugates thereof, are also provided.

FIELD OF THE INVENTION

The invention relates to derivatives of poly(ethylene glycol) and related hydrophilic polymers, to methods for their synthesis, and to surfaces and molecules modified by these polymers.

BACKGROUND OF THE INVENTION

Covalent attachment of the hydrophilic polymer poly(ethylene glycol), abbreviated PEG, also known as poly(ethylene oxide), abbreviated PEO, to molecules and surfaces is of considerable utility in biotechnology and medicine. In its most common form, PEG is a linear polymer terminated at each end with hydroxyl groups:

HO—CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—OH

The above polymer, alpha-, omega-dihydroxylpoly(ethylene glycol), can be represented in brief form as HO—PEG—OH where it is understood that the —PEG— symbol represents the following structural unit:

—CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—

where n typically ranges from about 3 to about 4000.

PEG is commonly used as methoxy-PEG—OH, or mPEG in brief, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification. The structure of mPEG is given below.

 CH₃O—(CH₂CH₂O)_(n)—CH₂CH₂—OH

The copolymers of ethylene oxide and propylene oxide are closely related to PEG in their chemistry, and they can be substituted for PEG in many of its applications.

HO—CH₂CHRO(CH₂CHRO)_(n)CH₂CH₂—OH

where R═H or alkyl, such as CH₃.

PEG is a polymer having the properties of solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule “conjugate” soluble. For example, it has been shown that the water-insoluble drug paclitaxel, when coupled to PEG, becomes water-soluble. Greenwald, et al., J. Org. Chem., 60:331-336 (1995).

To couple PEG to a molecule, such as a protein, it is often necessary to “activate” the PEG to prepare a derivative of the PEG having a functional group at the terminus. The functional group can react with certain moieties on the protein, such as an amino group, thus forming a PEG-protein conjugate. Many activated derivatives of PEG have been described. An example of such an activated derivative is the succinimidyl succinate “active ester”:

CH₃O—PEG—O₂C—CH₂CH₂—CO₂—NS

where NS═

Hereinafter, the succinimidyl active ester moiety will be represented as —CO₂—NS.

As applications of PEG chemistry have become more sophisticated, there has been an increasing need for heterofunctional PEGs, that is, PEGs bearing dissimilar terminal groups:

 X—PEG—Y

where X and Y are different groups. Such heterobifunctional PEGs bearing appropriate functional groups may be used to link the PEG to surfaces or biologically active molecules, with the other terminus attached, for example, to a biologically active molecule, a liposome, or a biosensor.

It is desirable in the biotechnical arts to continually develop activated polymers suitable for conjugation with one or more of various substances, including other polymers, peptides, proteins, carbohydrates, oligonucleotides, lipids, liposomes, cells, drugs, surfaces, and other biologically active moieties. Additionally, it would be advantageous to develop activated polymers that can be used for targeting or extended release formulations.

SUMMARY OF THE INVENTION

The invention utilizes hydroxyapatite surfaces, such as bone, for the delivery of biologically active agents with sustained lifetime within the body. Polyethylene glycol is often covalently attached to biologically active molecules to extend its circulation half-life, but the residence time of some conjugates remains suboptimal. There are many biologically active agents, both polypeptides and small drug molecules, that would benefit from the extended residence time within the body and targeting of hydroxyapatite surfaces, such as bone, provided by the invention described more filly below.

The invention provides an isolatable, activated hydroxyapatite-targeting polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a chemically reactive group or a protected chemically reactive group. For example, the hydroxyapatite-targeting moiety can be selected from the group consisting of tetracycline, calcein, bisphosphonates, polyaspartic acid, polyglutamic acid, and aminophosphosugars. The chemically reactive group or protected chemically reactive group is preferably selected from the group consisting of hydroxyl, protected hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, and tresylate. The polymer backbone may comprise, for example, poly(alkylene glycol), poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrolidone), poly(hydroxypropylmethacrylamide), poly(α-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, and copolymers, terpolymers, derivatives and mixtures thereof.

By reacting the chemically reactive group with a biologically active agent, the hydroxyapatite-targeting polymers of the invention can be used to tether a biologically active agent to a surface, such as a bone surface. Methods for preparation of the hydroxyapatite-targeting polymers, and biologically active conjugates thereof, are also provided.

In one embodiment, the invention provides a method of utilizing a bone surface in a bone-containing organism, such as a mammal, as a reservoir for a releasable biologically active agent. The method includes providing a hydroxyapatite-targeting, biologically active polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a biologically active agent through a linker, wherein at least one of the polymer backbone and the linker comprise a hydrolytically or enzymatically degradable linkage. A therapeutically effective amount of the polymeric structure is administered to a bone-containing organism such that at least a portion of the polymeric structure is bonded to a bone surface by the hydroxyapatite-targeting moiety. Preferably, the hydrolytically or enzymatically degradable linkage is selected from the group consisting of carbonate, carboxylate ester, phosphoester, orthoester, acetal, carbamate, disulfide, and peptide. The hydroxyapatite-targeting polymeric structure with the releasable biologically active agent will initially target bone or bone marrow surfaces within the organism, thereby using the bone surface as a reservoir or depot. The biologically active agent will be released into the organism over time as the degradable linkage degrades.

Thus, the invention allows a biologically active agent to be anchored to a hydroxyapatite surface in vivo and delivered over time to other parts of the organism for treatment of disease. In this manner, the residence time of the biologically active agent could be extended and the efficacy of the treatment improved. In addition, the activated polymer derivatives of the invention are isolatable such that the polymers can be separated and purified prior to attachment to a biologically active agent, thereby increasing yield and purity of the biologically active polymers.

DETAILED DESCRIPTION OF THE INVENTION

The terms “functional group”, “active moiety”, “activating group”, “reactive site”, “chemically reactive group” and “chemically reactive moiety” are used in the art and herein to refer to distinct, definable portions or units of a molecule. The terms are somewhat synonymous in the chemical arts and are used herein to indicate that the portions of molecules that perform some function or activity and are reactive with other molecules.

The term “linkage” or “linker” is used herein to refer to groups or bonds that normally are formed as the result of a chemical reaction and typically are covalent linkages. Hydrolytically stable linkages means that the linkages are substantially stable in water and do not react with water at useful pHs, e.g., under physiological conditions for an extended period of time, preferably indefinitely. Hydrolytically unstable or degradable linkages means that the linkages are degradable in water or in aqueous solutions, including for example, blood. Enzymatically unstable or degradable linkages means that the linkage can be degraded by one or more enzymes.

As used herein, the term “compound” is intended to refer to a chemical entity, whether in the solid, liquid or gaseous phase, and whether in a crude mixture or purified and isolated. The terms “alkyl,” “alkene,” and “alkoxy” include straight chain and branched alkyl, alkene, and alkoxy, respectively. The term “lower alkyl” refers to C₁-C₆ alkyl. The term “alkoxy” refers to oxygen substituted alkyl, for example, of the formulas —OR or —ROR¹, wherein R and R¹ are each independently selected alkyl. The terms “substituted alkyl” and “substituted alkene” refer to alkyl and alkene, respectively, substituted with one or more non-interfering substituents, such as but not limited to, C₃-C₆ cycloalkyl, e.g., cyclopropyl, cyclobutyl, and the like; acetylene; cyano; alkoxy, e.g., methoxy, ethoxy, and the like; lower alkanoyloxy, e.g., acetoxy; hydroxy; carboxyl; amino; lower alkylamino, e.g., methylamino; ketone; halo, e.g. chloro or bromo; phenyl; substituted phenyl, and the like. The term “halogen” includes fluorine, chlorine, iodine and bromine.

“Aryl” means one or more aromatic rings, each of 5 or 6 carbon atoms. Multiple aryl rings may be fused, as in naphthyl or unfused, as in biphenyl. Aryl rings may also be fused or unfused with one or more cyclic hydrocarbon, heteroaryl, or heterocyclic rings. “Substituted aryl” is aryl having one or more non-interfering groups as substituents.

“Non-interfering substituents” are those groups that yield stable compounds. Suitable non-interfering substituents or radicals include, but are not limited to, halo, C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₂-C₁₀ alkynyl, C₁-C₁₀ alkoxy, C₇-C₁₂ aralkyl, C₇-C₁₂ alkaryl, C₃-C₁₀ cycloalkyl, C₃-C₁₀ cycloalkenyl, phenyl, substituted phenyl, toluoyl, xylenyl, biphenyl, C₂-C₁₂ alkoxyalkyl, C₇-C₁₂ alkoxyaryl, C₇-C₁₂ aryloxyalkyl, C₆-C₁₂ oxyaryl, C₁-C₆ alkylsulfinyl, C₁-C₁₀ alkylsulfonyl, —(CH₂)_(m)—O—(C₁-C₁₀ alkyl) wherein m is from 1 to 8, aryl, substituted aryl, substituted alkoxy, fluoroalkyl, heterocyclic radical, substituted heterocyclic radical, nitroalkyl, —NO₂, —CN, —NRC(O)—(C₁-C₁₀ alkyl), —C(O)—(C₁-C₁₀ alkyl), C₂-C₁₀ thioalkyl, —C(O)O—(C₁-C₁₀ alkyl), —OH, —SO₂, ═S, —COOH, —NR₂, carbonyl, —C(O)—(C₁-C₁₀ alkyl)—CF₃, —C(O)—CF₃, —C(O)NR₂, —(C₁-C₁₀ alkyl)—S—(C₆-C₁₂ aryl), —C(O)—(C₆-C₁₂ aryl), —(CH₂)_(m)—O—(CH₂)_(m)—O—(C₁-C₁₀ alkyl) wherein each m is from 1 to 8, —C(O)NR₂, —C(S)NR₂, —SO₂NR₂, —NRC(O)NR₂, —NRC(S)NR₂, salts thereof, and the like. Each R as used herein is H, alkyl or substituted alkyl, aryl or substituted aryl, aralkyl, or alkaryl.

The term “amino acid” includes all essential and non-essential amino acids. Standard amino acid abbreviations known in the art are used herein.

The term “biologically active molecule”, “biologically active moiety” or “biologically active agent” when used herein means any substance which can affect any physical or biochemical properties of a biological organism, including but not limited to viruses, bacteria, fungi, plants, animals, and humans. In particular, as used herein, biologically active molecules include any substance intended for diagnosis, cure mitigation, treatment, or prevention of disease in humans or other animals, or to otherwise enhance physical or mental well-being of humans or animals. Examples of biologically active molecules include, but are not limited to, peptides, proteins, enzymes, small molecule drugs, dyes, lipids, nucleosides, oligonucleotides, cells, viruses, liposomes, microparticles and micelles. Classes of biologically active agents that are suitable for use with the invention include, but are not limited to, antibiotics, fingicides, anti-viral agents, anti-inflammatory agents, anti-tumor agents, cardiovascular agents, anti-anxiety agents, hormones, growth factors, steroidal agents, and the like.

The term “isolatable” is intended to mean that the activated polymers can be isolated or separated from other compounds prior to attachment to a biologically active agent. The acceptable level of isolation or purity will depend on various factors, such as the difficulty of purification, the type of contaminating compounds present, etc. By isolating or separating the hydroxyapatite-targeting activated polymers prior to attachment to the biologically active agent, better yield and purity is achieved. In addition, isolation of the polymer derivatives allows qualitative and quantitative analysis of the polymer prior to incorporation of the biologically active agent, which, in turn, increases the overall quality of the biologically active product. Having an isolatable hydroxyapatite-targeting activated polymer available for attachment to biologically active agents can result in a conjugate with a well-defined composition that can easily be conveyed to regulatory authorities. As exemplified in the examples below, the activated polymer derivatives are generally isolated and purified by precipitation followed by solvent extraction and/or chromatography techniques, such as ion-exchange chromatography.

The invention provides an isolatable hydroxyapatite-targeting polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a chemically reactive group, wherein the chemically reactive group is protected or unprotected.

The polymer backbone is a substantially non-immunogenic polymer, such as poly(ethylene glycol) (PEG). However, it should be understood that other related polymers are also suitable for use in the practice of this invention and that the use of the term PEG or poly(ethylene glycol) is intended to be inclusive and not exclusive in this respect. Preferably, the polymer backbone has from 2 to about 300 termini.

PEG is typically clear, colorless, odorless, soluble in water, stable to heat, inert to many chemical agents, does not hydrolyze or deteriorate, and is generally nontoxic. Poly(ethylene glycol) is considered to be biocompatible, which is to say that PEG is capable of coexistence with living tissues or organisms without causing harm. More specifically, PEG is non-immunogenic, which is to say that PEG does not tend to produce an immune response in the body. When attached to a molecule having some desirable function in the body, such as a biologically active agent, the PEG tends to mask the agent and can reduce or eliminate any immune response so that an organism can tolerate the presence of the agent. PEG conjugates tend not to produce a substantial immune response or cause clotting or other undesirable effects. PEG having the formula —CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—, where n is from about 3 to about 4000, preferably from about 3 to about 2000, is one useful polymer in the practice of the invention. Preferably, PEG having a molecular weight of from about 200 Da to about 100,000 Da is used as the polymer backbone.

The polymer backbone can be linear or branched. Branched polymer backbones are generally known in the art. Typically, a branched polymer has a central branch core moiety and a plurality of linear polymer chains linked to the central branch core. PEG is commonly used in branched forms that can be prepared by addition of ethylene oxide to various polyols, such as glycerol, pentaerythritol and sorbitol. The central branch moiety can also be derived from several amino acids, such as lysine. The branched polyethylene glycols can be represented in general form as R(—PEG—OH)_(m) in which R represents the core moiety, such as glycerol or pentaerythritol, and m represents the number of arms.

Many other polymers are also suitable for the invention. These polymers can be either in linear form or branched form, and include, but are not limited to, other poly(alkylene glycol), such as poly(propylene glycol) (“PPG”), copolymers of ethylene glycol and propylene glycol and the like, poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrrolidone), poly(hydroxypropylmethacrylamide), poly(α-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, and copolymers, terpolymers, derivatives and mixtures thereof. Although the molecular weight of each chain of the polymer backbone can vary, it is typically in the range of from about 100 Da to about 100,000 Da, preferably from about 6,000 Da to about 80,000 Da.

Those of ordinary skill in the art will recognize that the foregoing list for ubstantially water soluble non-immunogenic polymer backbone is by no means xhaustive and is merely illustrative, and that all polymeric materials having the qualities described above are contemplated.

The hydroxyapatite-targeting moiety may comprise any moiety capable of binding to, or otherwise exhibiting a chemical affinity for, hydroxyapatite surfaces (i.e. calcium phosphate), such as bone. The hydroxyapatite-targeting moiety is preferably capable of binding to any hydroxyapatite or calcium phosphate surface. In one embodiment, the hydroxyapatite-targeting moiety is selected from the group consisting of tetracycline, calcein, bisphosphonates, polyaspartic acid, polyglutamic acid, and aminophosphosugars. In a particularly preferred embodiment, the hydroxyapatite-targeting moiety is a bisphosphonate.

An example of a bisphosphonate suitable for use with the invention is shown below.

wherein Y and Z are independently selected from the group consisting of hydrogen, —OH, halogen, aryl, substituted aryl, pyridyl, furanyl, pyrrolidinyl, imidazonyl, C1-C30 alkyl, C1-C30 substituted alkyl, NH₂, NHR′, NR′₂, SH, and SR′, where R′ is C1-C30 alkyl, C1-C10 alkoxy, aryl or substituted aryl, and W is selected from the group consisting of hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, Na⁺, and K⁺. Preferably, Y is —NH(CH₂)_(p)—, where p is about 2 to about 6, Z is —OH, and each W is hydrogen. A particularly preferred bisphosphonate is 3-amino-1-hydroxypropane-1,1-diphosphonic acid.

Bisphosphonates are characterized by two carbon-phosphorous bonds, the carbon atom replacing the oxygen in the P—O—P (phosphorous-oxygen-phosphorous) bond of pyrophosphate and the P—C—P bond conferring resistance to chemical and enzymatic hydrolysis. Different substitutions on the carbon atom have created several different bisphosphonates, each with its own pharmacological properties. Etidronate, which contains a hydroxyl and methyl group substitution on the carbon atom, was the first bisphosphonate with a half-life in bone of greater than 90 days to be used therapeutically. Other more potent bisphosphonates have subsequently been developed, such as alendronate, which has an alkyl amine and hydroxyl group substitution on the carbon atom.

Bisphosphonates have a strong affinity for hydroxyapatite crystals and, in fairly high doses, inhibit calcification of bone in vivo by physicochemical mechanisms. Bisphosphonates are not metabolized, and seem to be absorbed, excreted, and stored unchanged. However, the side chains of some analogs of bisphosphonates may be modified. Plasma clearance is rapid (half-life around 2 hours) because of the rapid uptake of 20-60% of the absorbed fraction into the skeleton. The remainder is excreted in the urine. The half-life in bone is very long, with release of bisphosphonates occurring only after resorption of bone into which the compounds have been taken up.

The strong affinity of bisphosphonates, and other hydroxyapatite-targeting moieties, for bone enables the polymers of the invention to be used as hydroxyapatite-targeting delivery systems for biologically active agents. Preferably, the isolatable, activated hydroxyapatite-targeting polymers can be efficiently attached to the biologically active agent in a single step.

As noted above, the hydroxyapatite-targeting polymeric structure of the invention has at least one terminus bonded to a protected or unprotected chemically reactive group. The chemically reactive group is preferably suitable for attachment to a finctional group on a biologically active agent. Examples of suitable chemically reactive groups include hydroxyl, protected hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, and tresylate.

As would be understood in the art, the term “protected” refers to the presence of a protecting group or moiety that prevents reaction of the chemically reactive group under certain reaction conditions. The protecting group will vary depending on the type of chemically reactive group being protected. For example, if the chemically reactive group is an amine or a hydrazide, the protecting group is preferably selected from the group of tert-butyloxycarbonyl (t-Boc) and 9-fluorenylmethoxycarbonyl (Fmoc). If the chemically reactive group is a thiol, the protecting group is preferably orthopyridyldisulfide. If the chemically reactive group is a carboxylic acid, such as butanoic or propionic acid, or a hydroxyl group, the protecting group is preferably benzyl. Other protecting groups known in the art may also be used in the invention.

In one embodiment, the chemically reactive groups are selected from the group consisting of hydroxyl, protected hydroxyl, amine, protected amine, carboxylic acid, protected carboxylic acid, maleimide, active carbonates, such as N-hydroxysuccinimidyl carbonates and 1-benzotriazolyl carbonates, and active esters, such as N-hydroxysuccinimidyl esters and 1-benzotriazolyl esters.

One useful hydroxyapatite-targeting polymer has the following general structure:

Q—POLY—L—T

wherein POLY is a water-soluble and non-peptidic polymer, Q is a protected or unprotected chemically reactive group, L is a linker, and T is a hydroxyapatite-targeting moiety. Preferably, POLY is a poly(alkylene glycol), such as poly(ethylene glycol), having an average molecular weight from about 200 Da to about 100,000 Da, and T is a bisphosphonate. The linker L is the residue of the functional group used to attach the hydroxyapatite-targeting moiety to the polymer backbone. For example, the linker L may be a hydrolytically stable linkage selected from the group consisting of ether linkages, thio-ether linkages, amide linkages, amine linkages, urea linkages, or carbamate linkages. The linker L may also be a degradable linkage as described in more detail below.

In another embodiment, the hydroxyapatite-targeting polymer has the following general structure:

Q—PEG—L—T

wherein PEG is a poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da, Q is a protected or unprotected chemically reactive group, L is a linker, and T is a bisphosphonate. Advantageously, PEG is —CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—, wherein n is about 3 to about 2000.

In a third embodiment, the hydroxyapatite-targeting polymer has the following general structure:

T—L—POLY_(a)—R(—POLY_(b)—X)_(q)

wherein POLY_(a) and POLY_(b) are water-soluble and non-peptidic polymer backbones that may be the same or different;

each X is independently selected from the group consisting of alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, hydroxyl, protected hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, tresylate, and —L—T, with the proviso that at least one X is not —L—T;

R is a central core molecule, such as an amino acid or a polyol;

L is a linker;

T is a hydroxyapatite-targeting moiety; and

q is an integer from 2 to about 300.

Preferably, POLY_(a) and POLY_(b) are both poly(ethylene glycol) and R is selected from the group consisting of trimethylolpropane, di-rimethylolpropane, glycerol, pentaerythritol, sorbitol, lysine, and di-lysine.

Some examples of preferred embodiments of the hydroxyapatite-targeting polymers of the invention are provided as follows:

PEG(2,000)-α-amine-ω-AHPDP,

PEG(2,000)-α-N-maleimido-ω-AHPDP,

PEG(2,000)-α-AHPDP-ω-propionic acid,

PEG(2,000)-α-AHPDP-ω-propionic acid, N-hydroxysuccinimide ester,

PEG(2,000)-α-AHPDP-ω-protein,

PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP, and

PEG(10,000)-(α-AHPDP)₄

where —AHPDP represents 3-amino-1-hydroxypropane-1,1-diphosphonic acid.

The invention also includes conjugates of the hydroxyapatite-targeting polymers of the invention and biologically active agents. For example, the invention provides hydroxyapatite-targeting, biologically active polymers of the following general structure:

D—L′—POLY—L—T

wherein POLY is a water-soluble and non-peptidic polymer, D is a biologically active agent, L and L′ are linkers which may be the same or different, and T is a hydroxyapatite-targeting moiety.

In another embodiment, the invention provides hydroxyapatite-targeting, biologically active polymers of the following general structure:

D—L′—PEG—L—T

wherein PEG is a poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da, D is a biologically active agent, L and L′ are linkers which may be the same or different, and T is a bisphosphonate.

Further, the invention includes hydroxyapatite-targeting, biologically active polymers of the following general structure:

T—L—POLY_(a)—R(—POLY_(b)—L′—A)_(q)

wherein POLY_(a) and POLY_(b) are water-soluble and non-peptidic polymer backbones that may be the same or different;

each A is independently selected from the group consisting of hydroxyapatite-targeting moieties and biologically active agents, with the proviso that at least one A is a biologically active agent;

R is a central core molecule such as an amino acid or polyol;

L and L′ are linkers which may be the same or different;

T is a hydroxyapatite-targeting moiety; and

q is an integer from 2 to about 300.

A method of utilizing a hydroxyapatite surface, such as bone, as a reservoir for a releasable biologically active agent is also provided by the invention. The method includes providing a hydroxyapatite-targeting, biologically active polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a biologically active agent through a linker, wherein at least one of the polymer backbone and the linker comprises a hydrolytically or enzymatically degradable linkage. For example, in any of the structures given above for the biologically active polymers, the L, L′, POLY or PEG moieties may include a hydrolytically or enzymatically degradable linkage therein. For example, PEG can be prepared with ester linkages in the polymer backbone that are subject to hydrolysis. This hydrolysis results in cleavage of the polymer into fragments of lower molecular weight, as shown below.

—PEG—CO₂—PEG+H₂O—PEG—CO₂H+HO—PEG—

Preferably, the hydrolytically or enzymatically degradable linkage is selected from the group consisting of imine, carbonate, carboxylate ester, phosphoester, orthoester, acetal, carbamate linkages of the formula

where Ar is an aryl group, disulfide, and peptide. However, other degradable linkages known in the art may be used.

A therapeutically effective amount of the polymeric structure is administered to, for example, a bone-containing mammal, such that at least a portion of the polymeric structure is bonded to a bone surface by the hydroxyapatite-targeting moiety. The hydroxyapatite-targeting polymeric structure, with the releasable biologically active agent attached thereto, will initially target bone surfaces within the organism, thereby using the bone surface as a reservoir or depot. Due to the presence of the degradable linkage in either the polymer backbone or the linker, the biologically active agent will be released into the organism over time as the linkage degrades. In this manner, the biologically active agent is delivered to other parts of the organism for treatment of disease or other medical conditions. The released biologically active agent may be in native form, or attached to a linker or the linker and a fragment of the polymer backbone, depending on the placement of the degradable linkage. By tethering or anchoring the biologically active agent to bone, the residence time of the biologically active agent can be increased, which can lead to increased treatment efficacy.

The “therapeutically effective amount” will depend upon a number of factors, including the nature and severity of the condition or disease being treated, the type of biologically active agent being used, the size, age and general health of the subject, and other factors.

The polymeric structure may be administered by various routes, including oral, pulmonary, intravenous, subcutaneous, intramuscular, buccal, nasal, ocular, and rectal. The polymeric structure may also be administered with one or more pharmaceutically acceptable carriers, excipients or diluents.

The hydroxyapatite-targeting polymers of the invention can also be used to bind to other calcium phosphate surfaces or coatings, including hydroxyapatite-coated prosthetic devices and calcium phosphate particles. For example, the hydroxyapatite-targeting polymers of the invention can be used to prevent protein and cell adsorption into prosthetic devices or as part of a synthetically produced delivery device for biologically active agents.

The invention also includes methods of preparing the hydroxyapatite-targeting polymers, and biologically active conjugates thereof, described above. In a preferred embodiment, the activated polymers are prepared by providing a polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus covalently bonded to a first protected chemically reactive group and a second terminus covalently bonded to a second chemically reactive group selected from the group consisting of active carbonates and active esters. Examples of suitable active carbonates and active esters include N-hydroxysuccinimidyl esters, 1-benzotriazolyl esters, N-hydroxysuccinimidyl carbonates and 1-benzotriazolyl carbonates. The second chemically reactive group is reacted with a hydroxyapatite-targeting moiety to form a hydroxyapatite-targeting polymeric structure. The first protected chemically reactive group is preferably selected from the group consisting of protected hydroxyl, protected amine, protected carboxylic acid, protected hydrazide, and protected thiol. In this manner, at least one terminus of the polymer is protected such that the hydroxyapatite-targeting moiety does not attach thereto. This ensures that at least one terminus will be available for subsequent reaction with a biologically active agent. In one embodiment, the following polymer structure is used to react with the hydroxyapatite-targeting moiety:

X—O—CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—CO₂Y′

wherein X is an active carbonate or an active ester, Y′ is benzyl, and n is about 3 to about 2000. The method may further include deprotecting the first protected chemically reactive group so that the chemically reactive group is available for reaction, for example with a biologically active agent.

The method of producing a hydroxyapatite-targeting, biologically active polymer preferably includes providing a polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a chemically reactive group. The chemically reactive group is reacted with a biologically active agent such that the biologically active agent is attached to the second terminus of the polymeric structure to form a hydroxyapatite-targeting, biologically active polymeric structure. As would be understood by one of ordinary skill, the choice of chemically reactive group will depend on the available functional groups on the biologically active agent. For example, if the biologically active agent contains amine, hydroxyl, or thiol functional groups, an active carbonate or active ester may be used as the chemically reactive group. Particularly preferred chemically reactive groups useful for reacting with biologically active moieties include hydroxyl, amine, carboxylic acid, maleimide, active carbonates and active esters. The biologically active agent preferably comprises peptides, proteins, enzymes, small molecule drugs, dyes, nucleosides, oligonucleotides, lipids, phospholipids, cells, viruses, liposomes, microparticles or micelles. In one embodiment, the biologically active agent has at least one hydroxy group, such as quinidine, camptothecan, and paclitaxel.

The following examples are given to illustrate the invention, but should not be considered in limitation of the invention.

EXAMPLE 1 PEG(2,000)-α-N-CBZ-amine-ω-propionic Acid

To a solution of PEG(2,000)-α-amine-ω-propionic acid (5.0 g, 0.0025 moles) (Shearwater Polymers) in anhydrous methylene chloride (50 ml) N-(benzyloxycarbonyloxy) succinimide (0.80 g 0.0032 moles) and triethylamine (1.0 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the reaction mixture was washed with 20 ml phosphate buffer (4%, pH=3), dried with anhydrous magnesium sulfate and added to 400 ml cold diethyl ether. The precipitated product was filtered off and dried under reduced pressure. Yield 4.3 g. NMR (d₆-DMSO): 2.44 ppm (t, —CH₂—OOC—), 3.14 ppm (q, —CH₂NH—(C═O)—O—), 3.51 ppm (s, PEG backbone), 5.01 ppm (s, —CH₂—( benzyl)), 7.26 ppm (t, —NH—(C═O)), 7.33 ppm (m, —C₆H₅ (benzyl)).

EXAMPLE 2 PEG(2.000)-α-N-CBZ-amine-ω-propionic Acid, N-hydroxysuccinimide Ester

To a solution of PEG(2,000)-α-N-CBZ-amine-ω-propionic acid (4.30 g, 0.00183 moles) in anhydrous methylene chloride (50 ml), N-hydroxysuccinimide (0.22 g, 0.00192 moles) was added following by 1,3-dicyclohexylcarbodiimide (1.0 M solution in methylene chloride, 1.92 ml, 0.00192 moles). The reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the mixture was filtered and solvent was evaporated to dryness. The crude product was dissolved in methylene chloride and precipitated with isopropyl alcohol. The wet product was dried under reduced pressure.

Yield 3.5 g. NMR (d₆-DMSO): 2.81 ppm (s, —CH₂-CH₂—(succinate)), 2.92 ppm (t, —CH₂—COO—), 3.14 ppm (q, —CH ₂NH—(C═O)—O—), 3.51 ppm (s, PEG backbone), 5.01 ppm (s, —CH₂—( benzyl)), 7.26 ppm (t, —NH—(C═O)), 7.33 ppm (m, —C₆H₅ (benzyl)).

EXAMPLE 3 PEG(2,000)-α-amine-ω-AHPDP

To a solution of PEG(2,000)-α-N-CBZ-amine-ω-propionic acid, N-hydroxysuccinimide ester (3.0 g, 0.00046 moles) in acetonitrile (20 ml), AHPDP-2Bu₄N (0.93 g) and triethylamine (0.25 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next solvent was evaporated to dryness. The crude product was dissolved in DI water (50 ml) and filtered through Amberlite IR 120 (plus) column (50 ml). The pH of the solution was adjusted to 6.5 with 0.5 M sodium hydroxide and palladium on active carbon (10%) (0.6 g) was added. The mixture was hydrogenated overnight at room temperature under 40 psi of hydrogen. Next the mixture was filtered and water was distilled off under reduced pressure. The wet product was dissolved in methylene chloride (50 ml) then the solvent was distilled off. Finally the product was dried under reduced pressure. Yield 2.3 g.

NMR (d₆-DMSO): 1.98 ppm (m, —CH₂—(AHPDP)), 2.29 ppm (t, —CH₂—CO—), 2.94 ppm (m, —CH₂—amine), 3.51 ppm (s, PEG backbone), 7.88 ppm (t, —(C═O)—NH—).

EXAMPLE 4 PEG(2,000)-α-N-Maleimido-ω-AHPDP

To a solution of PEG(2,000)-α-amine-ω-AHPDP (2.0 g, 0.00046 moles) in acetonitrile (20 ml), β-maleimidopropionic acid N-hydroxysuccinimide ester (0.93 g) and triethylamine (0.25 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next solvent was evaporated to dryness. The crude product was dissolved in methylene chloride and precipitated with isopropyl alcohol. The wet product was dried under reduced pressure. Yield 1.8 g.

NMR (d₆-DMSO): 1.98 ppm (m, —CH₂—(AHPDP)), 2.29 ppm (t, —CH₂—CO—), 3.51 ppm (s, PEG backbone), 7.01 ppm (s, —CH2═CH2—, maleimide), 7.88 ppm (t, —(C═O)—NH—AHPDP), 8.01 ppm (t, —(C═O)—NH—PEG).

EXAMPLE 5 PEG(2,000)-α-hydroxy-ω-propionic Acid, Benzyl Ester

To a solution of PEG(2,000)-α-hydroxy-ω-propionic acid (10 g, 0.0050 moles) (Shearwater Polymers) in anhydrous methylene chloride (100 ml) 1-hydroxybenzotriazole (0.30 g), 4—(dimethylamino)pyridine (1.0 g), benzyl alcohol (10.8 g, 0.100 moles) and 1,3-dicyclohexylcarbodiimide (1.0 M solution in methylene chloride, 7.5 ml, 0.0075 moles) were added. The reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the mixture was concentrated to about 50 ml, filtered and added to 800 ml cold diethyl ether. The precipitated product was filtered off and dried under reduced pressure. Yield 8.2 g.

NMR (d₆-DMSO): 2.60 ppm (t, —CH₂—COO—), 3.51 ppm (s, PEG backbone), 4.57 ppm (t, —OH—), 5.11 ppm (s, —CH₂—(benzyl)), 7.36 ppm (m, —C₆H₅ (benzyl)).

EXAMPLE 6 PEG(2,000)-α-benzotriazole Carbonate-ω-propionic Acid, Benzyl Ester

To a solution of PEG(2,000)-α-hydroxy-ω-propionic acid, benzyl ester (8.2 g, 0.0025 moles) in acetonitrile (82 ml), pyridine (0.98 ml) and di(1-benzotriazolyl)carbonate (1.48 g) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the mixture was filtered and solvent was evaporated to dryness. The crude product was dissolved in methylene chloride and precipitated with isopropyl alcohol. The wet product was dried under reduced pressure. Yield 6.8 g.

NMR (d₆-DMSO): 2.60 ppm (t, —CH₂—COO—), 3.51 ppm (s, PEG backbone), 4.62 ppm (m, —CH₂—O(C═O)—), 5.11 ppm (s, —CH₂—(benzyl)), 7.36 ppm (m, —C₆H₅ (benzyl)), 7.60-8.50 ppm (4 m, aromatic protons of benzotriazole).

EXAMPLE 7 PEG(2,000)-α-AHPDP-ω-propionic Acid

To a solution of PEG(2,000)-α-benzotriazole carbonate-ω-propionic acid, benzyl ester (5.7 g, 0.0025 moles) in acetonitrile (40 ml), 3-amino-1-hydroxypropane-1,1-diphosphonic acid, ditetrabutylammonium salt (AHPDP-2Bu₄N) (1.92 g) and triethylamine (0.60 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next, solvent was evaporated to dryness. The crude product was dissolved in DI water (100 ml) and filtered through Amberlite IR 120 (plus) column (50 ml). The pH of the solution was adjusted to 7.2 with 0.5 M sodium hydroxide and palladium on active carbon (10%) (0.6 g) was added. The mixture was hydrogenated overnight at room temperature under 45 psi of hydrogen. Next the mixture was filtered and water was distilled off under reduced pressure. The wet product was dissolved in methylene chloride (150 ml) then the solvent was distilled off. Finally the product was dried under reduced pressure. Yield 4.7 g.

NMR (d₆-DMSO): 2.02 ppm (m, —CH₂—(AHPDP)), 2.44 ppm (t, —CH₂—COO—), 3.51 ppm (s, PEG backbone), 4.03 ppm (m, —CH₂—O(C═O)—), 7.11 ppm (t, —(C═O)—NH—).

EXAMPLE 8 PEG(2,000)-α-AHPDP-ω-propionic Acid, N-hydroxysuccinimide Ester

To a solution of PEG(2,000)-α-AHPDP-ω-propionic acid (4.7 g, 0.0020 equivalents) in anhydrous methylene chloride (100 ml), N-hydroxysuccinimide (0.70 g, 0.0024 moles) was added following by 1,3-dicyclohexylcarbodiimide (1.0 M solution in methylene chloride, 2.4 ml, 0.0024 moles). The reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the mixture was filtered and solvent was evaporated to dryness. The crude product was dissolved in methylene chloride and precipitated with isopropyl alcohol. Finally the product was dried under reduced pressure.

Yield 3.6 g. NMR (d₆-DMSO): 2.02 ppm (m, —CH₂—(AHPDP)), 2.81 ppm (s, —CH₂—CH₂—(succinate)), 2.92 ppm (t, —CH₂—COO—), 3.51 ppm (s, PEG backbone), 4.03 ppm (m, —CH₂—O(C═O)—), 7.11 ppm (t, —(C═O)—NH—).

EXAMPLE 9 Coupling of PEG(2,000)-α-AHPDP-ω-propionic Acid, N-hydroxysuccinimide Ester to Proteins

Lysozyme was used as a model protein, but any protein can be substituted in its place. Lysozyme (chicken egg white, MW=14,300 Da, 4 mg, 2.8E-7 moles) was dissolved in 2 ml of 50 mM sodium phosphate buffer, pH 7.2. For low degree of PEGylation, approximately 1.1 mg (5.6E-7 moles) of PEG(2,000)-α-AHPDP-ω-propionic acid, N-hydroxysuccinimide ester was added to the lysozyme solution and mixed at room temperature for 5 hours. For high degree of PEGylation, approximately 5.6 mg (2.8E-6 moles) of PEG(2,000)-α-AHPDP-ω-propionic acid, N-hydroxysuccinimide ester was added to the lysozyme solution and mixed at room temperature for 5 hours.

Each PEG-lysozyme multimer (1-PEGmer, 2-PEGmer, 3-PEGmer) was separated on a Superdex 75 size exclusion column (Amersham Pharmacia Biotech) with 5 mM sodium phosphate buffer, pH 7.2 as the elution buffer. Analysis of the purity of the multimers was determined by gel and capillary electrophoresis. Each purified multimer was used to determine the efficiency of conjugate binding to hydroxyapatite.

EXAMPLE 10 Bindinz of PEG(2,000)-α-AHPDP-ω-protein to Hydroxyapatite In Vitro

Hydroxyapatite (Fast Flow, Fluka, 120 mg) was equilibrated in 1 ml of 5 mM sodium phosphate buffer, pH 7.2 for 24 hours at 37° C. Native lysozyme or the PEG-lysozyme conjugate was mixed with the hydroxyapatite suspension in a 1:1 volume ratio and stirred at 37° C. Samples were taken from the mixture at various time points and analyzed for the percentage of unbound protein or PEG-protein conjugate by gel permeation chromatography. Native lysozyme did not adsorb to hydroxyapatite while 100% of the 3-PEGmer and 2-PEGmer adsorbed by 1½ hour incubation. The 1-PEGmer conjugate had limited adsorption after 1½ hour incubation. All samples had similar protein concentrations.

EXAMPLE 11 PEG(2,000)-α-methoxy-ω-_(L)-tyrosine

To a solution of PEG(2,000)-α-methoxy-ω-amine (2.0 g, 0.0010 moles)(Shearwater Polymers) in anhydrous acetonitrile (30 ml) N-CBZ-_(L)-tyrosine p-nitrophenyl ester (0.42 g 0.0010 moles) and triethylamine (0.3 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the solvent was distilled off under reduced pressure. The residue was dissolved in 200 ml DI water and the resulting solution was filtered through anion exchange resin (to remove p-nitrophenol) and palladium on active carbon (10%) (0.3 g) was added. The mixture was hydrogenated overnight at room temperature under 40 psi of hydrogen. Next the mixture was filtered and the product was extracted with methylene chloride. The extract was dried with anhydrous magnesium sulfate and solvent was distilled off under reduced pressure. The wet product was dried under reduced pressure. Yield 1.25 g.

NMR (d₆-DMSO): 2.62 ppm and 2.82 ppm (m, —CH₂—(tyrosine)), 3.23 ppm (s, —OCH₃), 3.51 ppm (s, PEG backbone), 4.12 ppm (m, CH—(tyrosine)), 6.64 ppm and 6.97 ppm (2 m, aromatic protons of tyrosine), 7.86 ppm (t, —(C═O)—NH—PEG).

EXAMPLE 12 PEG(2,000)-α-N-CBZ-_(L)-tyrosine-ω-propionic Acid

To a solution of PEG(2,000)-α-amino-ω-propionic acid (2.50 g, 0.0012 moles) (Shearwater Polymers) in anhydrous acetonitrile (30 ml) N-CBZ-_(L)-tyrosine p-nitrophenyl ester (0.54 g 0.0012 moles) and triethylamine (0.5 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the solvent was distilled off under reduced pressure. The residue was dissolved in 220 ml DI water and the resulting solution was filtered through anion exchange resin to remove p-nitrophenol. NaCl (20 g) was added and the pH of the solution was adjusted to 3.0 with 5% phosphoric acid. The product was extracted with methylene chloride. The extract was dried with anhydrous magnesium sulfate and solvent was distilled off under reduced pressure. The wet product was dried under reduced pressure. Yield 1.75 g.

NMR (d₆-DMSO): 2.44 ppm (t, —CH₂—COO—), 2.62 ppm and 2.82 ppm (m, —CH₂—(tyrosine)), 3.51 ppm (s, PEG backbone), 4.12 ppm (m, CH—(tyrosine)), 4.94 ppm (m, —CH₂—(benzyl)), 6.64 ppm and 7.04 ppm (2m, aromatic protons of tyrosine), 7.25 ppm (d, —(C═O)—NH-tyrosine), 7.32 ppm (m, —C₆H₅ (benzyl)), 8.01 ppm (t, —(C═O)—NH—PEG).

EXAMPLE 13 PEG(2,000)-α-N-CBZ-_(L)-tyrosine-ω-propionic Acid, N-hydroxysuccinimide Ester

To a solution of PEG(2,000)-α-N-CBZ-_(L)-tyrosine-ω-propionic acid (1.66 g, 0.00070 moles) in anhydrous methylene chloride (20 ml), N-hydroxysuccinimide (0.089 g, 0.00077 moles) was added following by 1,3-dicyclohexylcarbodiimide (1.0 M solution in methylene chloride, 0.77 ml, 0.00077 moles). The reaction mixture was stirred overnight at room temperature under argon atmosphere. Next the mixture was filtered and solvent was evaporated to dryness. The crude product was dissolved in methylene chloride and precipitated with isopropyl alcohol. The wet product was dried under reduced pressure. Yield 1.26 g.

NMR (d₆-DMSO): 2.62 ppm and 2.82 ppm (m, —CH₂—(tyrosine)), 2.81 ppm (s, —CH₂—CH₂—(succinate)), 2.92 ppm (t, —CH₂—COO—), 3.51 ppm (s, PEG backbone), 4.12 ppm (m, CH—(tyrosine)), 4.94 ppm (m, —CH₂—(benzyl)), 6.64 ppm and 7.04 ppm (2 m, aromatic protons of tyrosine), 7.25 ppm (d, —(C═O)—NH-tyrosine), 7.32 ppm (m, —C₆H₅ (benzyl)), 8.01 ppm (t, —(C═O)NH—PEG).

EXAMPLE 14 PEG(2,000)-α-tyrosine-ω-AHPDP

To a solution of PEG(2,000)-α-N-CBZ-_(L)-tyrosine-ω-propionic acid, N-hydroxysuccinimide ester (1.15 g, 0.00046 moles) in acetonitrile (15 ml), AHPDP-2Bu₄N (0.36 g) and triethylamine (0.10 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next solvent was evaporated to dryness. The crude product was dissolved in DI water (50 ml) and filtered through Amberlite IR 120 (plus) column (20 ml). The pH of the solution was adjusted to 6.5 with 0.5 M sodium hydroxide and palladium on active carbon (10%) (0.3 g) was added. The mixture was hydrogenated overnight at room temperature under 40 psi of hydrogen. Next the mixture was filtered and water was distilled off under reduced pressure. The wet product was dissolved in methylene chloride (50 ml) then the solvent was distilled off. Finally the product was dried under reduced pressure. Yield 0.65 g.

NMR (d₆-DMSO): 2.02 ppm (m, —CH₂—(AHPDP)), 2.29 ppm (t, —CH₂—CO—), 3.51 ppm (s, PEG backbone), 4.13 ppm (m, —CH—(tyrosine)), 6.70 ppm and 7.01 ppm (2 m, aromatic protons of tyrosine), 7.94 ppm (d, —(C═O)—NH—AHPDP), 8.01 ppm (t, —(C═O)—NH—PEG).

EXAMPLE 15 Binding of PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP and Other PEG-AHPDP Derivatives to Hydroxyapatite In Vitro

Hydroxyapatite (Fast Flow, Fluka, 12 mg) was equilibrated in 1 ml of 5 mM sodium phosphate buffer, pH 7.2 for 24 hours at 37° C. AHPDP, PEG(2,000)-α-methoxy-ω-hydroxy, PEG(2,000)-α-methoxy-ω-_(L)-tyrosine, PEG(5,000)-α-amine-ω-AHPDP, and PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP derivatives at concentrations ranging from 0.1 mg/ml to 1.0 mg/ml in 5 mM sodium phosphate buffer, pH 7.2 were individually mixed with the hydroxyapatite suspension in a 1:1 volume ratio and stirred at 37° C. Samples were taken from each mixture at various time points and analyzed for the percentage of unbound PEG or PEG-AHPDP derivative by gel permeation chromatography. PEG(2,000)-α-methoxy-ω-hydroxy and PEG(2,000)-α-methoxy-ω-_(L)-tyrosine did not show any adsorption to hydroxyapatite while the PEG-AHPDP derivatives showed 100% adsorption by 8 hours of incubation. AHPDP had an adsorption half-life of around 1 min while the PEG(5,000)-α-amine-ω-AHPDP and PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP derivatives had a 45 minute and 19 minute adsorption half-life, respectively.

EXAMPLE 16 Excretion Study of PEG(2,000)-α-_(L)-tyrosine^(I125)-ω-AHPDP in Mice

PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP and mPEG(2,000)-α-_(L)-tyrosine were iodinated using a standard chloramine-T procedure (Bolton, Methods Enzymol. 124: 18-29, 1986). In brief, a 15 μL aliquot of PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP or mPEG(2,000)-α-_(L)-tyrosine (2 μg/μL total weight) was mixed with 50 μL of 5 μmM phosphate buffer. To this was added 1 mCi of NaI¹²⁵ and 10 μL of Chloramine-T (1 μg/μL). The mixture was finger vortexed for 30s followed by addition of 100 μL of L-Cys (2 μg/μL), followed by another 30s of finger vortexing. The purification was carried out by reverse-phase chromatography. The samples were eluted at 37° C. using a curvilinear gradient of 0.1% trifluroacetic acid (TFA) in acetonitrile (19-63%) versus 0.1% aqueous TFA over 30 min at 1.5 ml.min⁻¹.

Male ICR mice (20-25 g) were administered either 2 μCi of PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP or mPEG(2,000)-α-_(L)-tyrosine via tail vein injection (100 μL; i.v.). The mice were placed into metabolism cages and urine and feces were collected at the following time points (1, 2, 3, 4, 6, & 8 hr post-injection) (n=6). The samples were counted on a gamma counter and the counts were converted to a percentage of total counts injected. mPEG(2,000)-α-_(L)-tyrosine had an average % [¹²⁵ I] captured of 47.5 with greater than 95% of radiolabeled compound being excreted in the urine. PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP had a significantly lower percentage (5-fold) of [¹²⁵ I] captured (7.7 %) during the 8 hour time course than the mPEG(2,000)-α-_(L)-tyrosine and again ˜95% of the radiolabeled compound was excreted in the urine. The two compounds produced similar excretion profiles over the 8-hour period with the exception that PEG(2,000)-α-_(L)-tyrosine-ω-AHPDP produced a significantly greater percentage of [¹²⁵ I] retained in the body.

EXAMPLE 17 PEG(10,000)-(α-AHPDP)₄

To a solution of PEG(10,000)-(α-benzotriazole carbonate)₄ (Shearwater Polymers) (4.2 g, 0.00042 moles) in acetonitrile (40 ml), 3-amino-1-hydroxypropane-1,1-diphosphonic acid, ditetrabutylammonium salt (AHPDP-2Bu₄N) (1.32 g) and triethylamine (0.4 ml) were added and the reaction mixture was stirred overnight at room temperature under argon atmosphere. Next, solvent was evaporated to dryness. The crude product was dissolved in DI water (100 ml) and filtered through Amberlite IR 120 (plus) column (50 ml). Next the water was distilled off under reduced pressure. The wet product was dissolved in methylene chloride (150 ml) then the solvent was distilled off. Finally the product was dried under reduced pressure. Yield 3.1 g.

NMR (d₆-DMSO): 2.02 ppm (m, —CH₂—(AHPDP)), 3.51 ppm (s, PEG backbone), 4.03 ppm (m, —CH₂—O(C═O)—), 7.11 ppm (t, —(C═O)—NH—).

EXAMPLE 18 PEG(3,400)-α-CBZ-amine-ω-propionic Acid, Quinidine Ester

PEG(3,400)-α-CBZ-amine-ω-propionic acid (Shearwater Corporation) (3.40 g, ˜1.00 mmol), quinidine (0.49 g, ˜1.50 mmol), HOBT (catalytic amount), and DMAP (0.15 g, ˜1.23 mmol) were dissolved in CH₂Cl₂ (100 ml). The solution was treated with DCC (0.31 g, ˜1.50 mmol) that was dissolved in CH₂Cl₂ (3 ml) and was allowed to stir at room temperature under argon overnight (˜8 h). The solvent was removed in vacuo and the residual syrup was dissolved in toluene (25 ml) and filtered through a plug of celite. The toluene was removed in vacuo and the syrup was dissolved in CH₂Cl₂ (5 ml). Addition to diethyl ether yielded the product as a white precipitate, which was collected by filtration and dried under vacuum (Yield 2.77 g, ˜81%).

EXAMPLE 19 PEG(3.400)-α-amine-ω-propionic Acid, Quinidine Ester

PEG(3,400)-α-CBZ-amine-ω-propionic acid, quinidine ester (2.50 g) was dissolved in ethyl alcohol and palladium on active carbon (10%) (0.5 g) was added. The mixture was hydrogenated overnight at room temperature under 40 psi of hydrogen. Next the mixture was filtered and ethanol was distilled off under reduced pressure. Yield 2.1 g. NMR spectrum of the product showed that the amine group connected to PEG backbone was completely deprotected.

EXAMPLE 20 PEG(5,400)-α-AHPDP-ω-propionic Acid, Quinidine Ester

To a solution of PEG(2,000)-α-AHPDP-ω-propionic acid, NHS ester (1.22 g, 0.00050 moles) in anhydrous methylene chloride (100 ml), PEG(3,400)-α-amine-ω-propionic acid, quinidine ester( 1.89 g, 0.00050 moles) was added followed by triethylamine (0.10 ml). The mixture was stirred overnight under argon atmosphere. Next the reaction product was precipitated with ethyl ether and dried under reduced pressure. Yield 3.01 g Gel permeation chromatography showed that product with molecular weight 5,700 Da was formed (98%).

Hydrolysis half-life of the ester in PEG(5,400)-α-AHPDP-ω-propionic acid, quinidine ester in buffer at pH 7.3 is 239h (@22° C.) and 46h (@37° C.) leading to the release of quinidine. 

What is claimed is:
 1. An isolatable hydroxyapatite-targeting polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a chemically reactive group, wherein said chemically reactive group is protected or unprotected.
 2. A polymeric structure according to claim 1, wherein said hydroxyapatite-targeting moiety is selected from the group consisting of tetracycline, calcein, bisphosphonates, polyaspartic acid, polyglutamic acid, and aminophosphosugars.
 3. A polymeric structure according to claim 1, wherein said hydroxyapatite-targeting moiety is a bisphosphonate.
 4. A polymeric structure according to claim 3, wherein the bisphosphonate has the following structure:

wherein Y and Z are independently selected from the group consisting of hydrogen, —OH, halogen, aryl, substituted aryl, pyridyl, furanyl, pyrrolidinyl, imidazonyl, C1-C30 alkyl, C1-C30 substituted alkyl, NH₂, NHR′, NR′₂, SH, and SR′, where R′ is C1-C30 alkyl, C1-C10 alkoxy, aryl or substituted aryl, and W is selected from the group consisting of hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, Na⁺, and K⁺.
 5. A polymeric structure according to claim 4, wherein Y is —NH(CH₂)p—, p is about 2 to about 6, Z is —OH, and each W is hydrogen.
 6. A polymeric structure according to claim 1, wherein said chemically reactive group is selected from the group consisting of hydroxyl, protected hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, and tresylate.
 7. A polymeric structure according to claim 1, wherein said chemically reactive group is a protected amine or a protected hydrazide, wherein the protecting group is selected from the group of tert-butyloxycarbonyl and 9-fluorenylmethoxycarbonyl.
 8. A polymeric structure according to claim 1, wherein said chemically reactive group is a protected thiol, wherein the protecting group is orthopyridyldisulfide.
 9. A polymeric structure according to claim 1, wherein said chemically reactive group is a protected carboxylic acid or a protected hydroxyl, wherein the protecting group is benzyl.
 10. A polymeric structure according to claim 1, wherein said chemically reactive group is selected from the group consisting of hydroxyl, protected hydroxyl, amine, protected amine, carboxylic acid, protected carboxylic acid, maleimide, active carbonates and active esters.
 11. A polymeric structure according to claim 1, wherein said polymer backbone is selected from the group consisting of poly(alkylene glycol), poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrolidone), poly(hydroxypropylmethacrylamide), poly(α-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, and copolymers, terpolymers, derivatives and mixtures thereof.
 12. A polymeric structure according to claim 1, wherein said polymer backbone is poly(alkylene glycol).
 13. A polymeric structure according to claim 12, wherein said polymer backbone is poly(ethylene glycol).
 14. A polymeric structure according to claim 12, wherein the poly(alkylene glycol) backbone comprises a hydrolytically or enzymatically degradable linkage.
 15. A polymeric structure according to claim 14, wherein the degradable linkage is selected from the group consisting of imine, carbonate, carboxylate ester, phosphoester, orthoester, acetal, carbamate, disulfide, and peptide.
 16. A polymeric structure according to claim 1, wherein said polymer backbone has from 2 to about 300 termini.
 17. A polymeric structure according to claim 1, wherein the polymeric structure has the following structure: Q—POLY—L—T wherein POLY is a water-soluble and non-peptidic polymer, Q is a protected or unprotected chemically reactive group, L is a linker, and T is a hydroxyapatite-targeting moiety.
 18. A polymeric structure according to claim 17, wherein POLY is selected from the group consisting of poly(alkylene glycol), poly(oxyethylated polyol), poly(olefmic alcohol), poly(vinylpyrolidone), poly(hydroxypropylmethacrylamide), poly(α-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, and copolymers, terpolymers, derivatives and mixtures thereof.
 19. A polymeric structure according to claim 17, wherein L is selected from the group consisting of ether linkages, thio-ether linkages, amide linkages, amine linkages, urea linkages, and carbamate linkages.
 20. A polymeric structure according to claim 17, wherein POLY is a poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da.
 21. A polymeric structure according to claim 17, wherein T is selected from the group consisting of tetracycline, calcein, bisphosphonates, polyaspartic acid, polyglutamic acid, and aminophosphosugars.
 22. A polymeric structure according to claim 17, wherein T is a bisphosphonate.
 23. A polymeric structure according to claim 22, wherein the bisphosphonate has the following structure:

wherein Y and Z are independently selected from the group consisting of hydrogen, —OH, halogen, aryl, substituted aryl, pyridyl, furanyl, pyrrolidinyl, imidazonyl, C1-C30 alkyl, C1-C30 substituted alkyl, NH₂, NHR′, NR′₂, SH, and SR′, where R′ is C1-C30 alkyl, C1-C10 alkoxy, aryl or substituted aryl, and W is selected from the group consisting of hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, Na⁺, and K⁺.
 24. A polymeric structure according to claim 23, wherein Y is —NH(CH₂)p—, p is about 2 to about 6, Z is —OH, and each W is hydrogen.
 25. A polymeric structure according to claim 17, wherein Q is selected from the group consisting of hydroxyl, protected hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, and tresylate.
 26. A polymeric structure according to claim 17, wherein Q is selected from the group consisting of hydroxyl, protected hydroxyl, amine, protected amine, carboxylic acid, protected carboxylic acid, maleimide, active carbonates and active esters.
 27. A polymeric structure according to claim 1, having the following structure: Q—PEG—L—T wherein PEG is a poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da, Q is a protected or unprotected chemically reactive group selected from the group consisting of hydroxyl, protected hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, and tresylate, L is a linker, and T is a bisphosphonate.
 28. A polymeric structure according to claim 27, wherein PEG is —CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—, wherein n is about 3 to about
 2000. 29. A polymeric structure according to claim 27, wherein L is selected from the group consisting of ether linkages, thio-ether linkages, amide linkages, amine linkages, urea linkages, and carbamate linkages.
 30. A polymeric structure according to claim 1, having the following structure: T—L—POLY_(a)—R(—POLY_(b)—X)_(q) wherein POLY_(a) and POLY_(b) are water-soluble and non-peptidic polymer backbones that may be the same or different; each X is independently selected from the group consisting of alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, hydroxyl, protected hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, tresylate, and L—T, with the proviso that at least one X is not a hydroxyapatite-targeting moiety; R is a central core molecule; L is a linker; T is said hydroxyapatite-targeting moiety; and q is an integer from 2 to about
 300. 31. A polymeric structure according to claim 30, wherein POLY_(a) and POLY_(b) are poly(ethylene glycol).
 32. A polymeric structure according to claim 30, wherein R is selected from the group consisting of trimethylolpropane, di-trimethylolpropane, glycerol, pentaerythritol, sorbitol, lysine, and di-lysine.
 33. A polymeric structure according to claim 30, wherein L is selected from the group consisting of ether linkages, thio-ether linkages, amide linkages, amine linkages, urea linkages, and carbamate linkages.
 34. A hydroxyapatite-targeting, biologically active polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a biologically active agent through a linker, wherein at least one of the polymer backbone and the linker comprise a hydrolytically or enzymatically degradable linkage.
 35. A polymeric structure according to claim 34, wherein the degradable linkage is selected from the group consisting of imine, carbonate, carboxylate ester, phosphoester, orthoester, acetal, carbamate, disulfide, and peptide.
 36. A polymeric structure according to claim 34, wherein the polymer backbone comprises a hydrolytically or enzymatically degradable linkage.
 37. A polymeric structure according to claim 34, wherein the linker comprises a hydrolytically or enzymatically degradable linkage.
 38. A polymeric structure according to claim 34, having the following structure: D—L′—POLY—L—T wherein POLY is a water-soluble and non-peptidic polymer, D is a biologically active agent, L and L′ are linkers which may be the same or different, and T is a hydroxyapatite-targeting moiety, and wherein at least one of POLY, L, and L′ comprise a hydrolytically or enzymatically degradable linkage.
 39. A polymeric structure according to claim 38, wherein POLY is a poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da.
 40. A polymeric structure according to claim 38, wherein T is a bisphosphonate.
 41. A polymeric structure according to claim 34, having the following structure: T—L—POLY_(a)—R(—POLY_(b)—L′—A)_(q) wherein POLY_(a) and POLY_(b) are water-soluble and non-peptidic polymer backbones that may be the same or different; each A is independently selected from the group consisting of hydroxyapatite-targeting moieties and biologically active agents, with the proviso that at least one A is a biologically active agent; R is a central core molecule; L and L′ are linkers which may be the same or different; T is a hydroxyapatite-targeting moiety; q is an integer from 2 to about 300; and wherein at least one of POLY_(a), POLY_(b), L and L′ comprises a hydrolytically or enzymatically degradable linkage.
 42. A polymeric structure according to claim 41, wherein POLY_(a) and POLY_(b) are poly(ethylene glycol).
 43. A polymeric structure according to claim 34, having the following structure: D—L′—POLY—L—T wherein POLY is poly(ethylene glycol), D is a biologically active agent having at least one hydroxy group, L is a hydrolytically stable linkage, L′ is a hydrolytically degradable linkage, and T is a bisphosphonate moiety.
 44. A polymeric structure according to claim 43, wherein the biologically active agent is selected from the group consisting of quinidine, camptothecan, and paclitaxel.
 45. A polymeric structure according to claim 43, wherein L′ is selected from the group consisting of ester, carbamate, carbonate, and acetal.
 46. A polymeric structure according to claim 43, wherein L is selected from the group consisting of ether, thio-ether, amide, and carbamate.
 47. A method of preparing an isolatable hydroxyapatite-targeting polymeric structure, comprising: providing a polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus covalently bonded to a first protected chemically reactive group and a second terminus covalently bonded to a second chemically reactive group selected from the group consisting of active carbonates and active esters; and reacting the second chemically reactive group with a hydroxyapatite-targeting moiety to form a hydroxyapatite-targeting polymeric structure.
 48. A method according to claim 47, wherein the first protected chemically reactive group is selected from the group consisting of protected hydroxyl, protected amine, protected carboxylic acid, protected hydrazide, and protected thiol.
 49. A method according to claim 47, wherein the polymer backbone is selected from the group consisting of poly(alkylene glycol), poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrolidone), poly(hydroxypropyhnethacrylamide), poly(α-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, and copolymers, terpolymers, derivatives and mixtures thereof.
 50. A method according to claim 47, wherein the second chemically reactive group is selected from the group consisting of N-hydroxysuccinimidyl esters, 1-benzotriazolyl esters, N-hydroxysuccinimidyl carbonates and 1-benzotriazolyl carbonates.
 51. A method according to claim 47, wherein said providing step comprises providing a polymeric structure having the following structure: X—O—CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—CO₂Y′ wherein X is an active carbonate or an active ester, Y′ is benzyl, and n is about 3 to about
 2000. 52. A method according to claim 47, wherein the hydroxyapatite-targeting moiety is selected from the group consisting of tetracycline, calcein, bisphosphonates, polyaspartic acid, polyglutamic acid, and aminophosphosugars.
 53. A method according to claim 47, fturther comprising the steps of: deprotecting the first protected chemically reactive group; converting the deprotected first chemically reactive group to an active carbonate or an active ester; reacting the active carbonate or active ester with a biologically active agent such that the biologically active agent is attached to the polymeric structure.
 54. A method according to claim 53, wherein the biologically active agent is selected from a group consisting of peptides, proteins, enzymes, small molecule drugs, dyes, nucleosides, oligonucleotides, lipids, phospholipids, cells, viruses, liposomes, microparticles and micelles.
 55. A method of preparing a hydroxyapatite-targeting, biologically active polymeric structure, comprising: providing a polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two terimin, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a chemically reactive group, wherein the chemically reactive group is protected or unprotected; and reacting the chemically reactive group with a biologically active agent such that the biologically active agent is attached to the second terminus of the polymeric structure to form a hydroxyapatite-targeting, biologically active polymeric structure.
 56. A method according to claim 55, wherein the hydroxyapatite-targeting moiety is selected from the group consisting of tetracycline, calcein, bisphosphonates, polyaspartic acid, polyglutamic acid, and aminophosphosugars.
 57. A method according to claim 55, wherein the hydroxyapatite-targeting moiety is a bisphosphonate.
 58. A method according to claim 55, wherein the chemically reactive group is selected from the group consisting of hydroxyl, active ester, active carbonate, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, hydrazide, thiol, carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, and tresylate.
 59. A method according to claim 55, wherein the chemically reactive group is selected from the group consisting of hydroxyl, amine, carboxylic acid, maleimide, active carbonates and active esters.
 60. A method according to claim 55, wherein the polymer backbone is selected from the group consisting of poly(alkylene glycol), poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrolidone), poly(hydroxypropylmethacrylamide), poly(α-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, and copolymers, terpolymers, derivatives and mixtures thereof.
 61. A method according to claim 55, wherein said polymer backbone is poly(alkylene oxide).
 62. A method according to claim 55, wherein the hydroxyapatite-targeting, biologically active polymeric structure has the following structure: D—L′—POLY—L—T wherein POLY is a water-soluble and non-peptidic polymer, D is a biologically active agent, L and L′ are linkers which may be the same or different, and T is a hydroxyapatite-targeting moiety.
 63. A method according to claim 62, wherein POLY is selected from the group consisting of poly(alkylene glycol), poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrolidone), poly(hydroxypropylmethacrylamide), poly(α-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, and copolymers, terpolymers, derivatives and mixtures thereof.
 64. A method according to claim 62, wherein T is selected from the group consisting of tetracycline, calcein, bisphosphonates, polyaspartic acid, polyglutamic acid, and aminophosphosugars.
 65. A method according to claim 62, wherein L and L′ are independently selected from the group consisting of ether linkages, thio-ether linkages, amide linkages, amine linkages, urea linkages, and carbamate linkages.
 66. A method according to claim 55, wherein the hydroxyapatite-targeting, biologically active polymeric structure has the following structure: D—L′—PEG—L—T wherein PEG is a poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da, D is a biologically active, L and L′ are linkers which may be the same or different, and T is a bisphosphonate.
 67. A method according to claim 55, wherein the hydroxyapatite-targeting, biologically active polymeric structure has the following structure: T—L—POLY_(a)—R(—POLY_(b)—L′—A)_(q) wherein POLY_(a) and POLY_(b) are water-soluble and non-peptidic polymer backbones that may be the same or different; each A is independently selected from the group consisting of hydroxyapatite-targeting moieties and biologically active agents, with the proviso that at least one A is a biologically active agent; R is a central core molecule; L and L′ are linkers which may be the same or different; T is a hydroxyapatite-targeting moiety; and q is an integer from 2 to about
 300. 68. A method according to claim 67, wherein POLY_(a) and POLY_(b) are poly(ethylene glycol).
 69. A method according to claim 67, wherein R is selected from the group consisting of trimethylolpropane, di-trimethylolpropane, glycerol, pentaerythritol, sorbitol, lysine, and di-lysine.
 70. A method according to claim 55, wherein the biologically active agent is selected from a group consisting of peptides, proteins, enzymes, small molecule drugs, dyes, nucleosides, oligonucleotides, lipids, phospholipids, cells, viruses, liposomes, microparticles and micelles.
 71. A method according to claim 55, further comprising the step of bonding the hydroxyapatite-targeting moiety to a hydroxyapatite surface.
 72. A method of utilizing a bone surface in an organism as a reservoir for a releasable biologically active agent, comprising: providing a hydroxyapatite-targeting, biologically active polymeric structure comprising a linear or branched water-soluble and non-peptidic polymer backbone having at least two termini, a first terminus being covalently bonded to a hydroxyapatite-targeting moiety and a second terminus covalently bonded to a biologically active agent through a linker, wherein at least one of the polymer backbone and the linker comprise a hydrolytically or enzymatically degradable linkage; and administering a therapeutically effective amount of the polymeric structure to a bone-containing organism such that at least a portion of the polymeric structure is bonded to a bone surface by the hydroxyapatite-targeting moiety.
 73. A method according to claim 72, wherein the degradable linkage is selected rom the group consisting of imine, carbonate, carboxylate ester, phosphoester, rthoester, acetal, carbamate, disulfide, and peptide.
 74. A method according to claim 72, wherein the hydroxyapatite-targeting, biologically active polymeric structure has the following structure: D—L′—POLY—L—T wherein POLY is a water-soluble and non-peptidic polymer, D is a biologically active agent, L and L′ are linkers which may be the same or different, and T is a hydroxyapatite-targeting moiety, and wherein at least one of POLY, L and L′ comprises a hydrolytically or enzymatically degradable linkage.
 75. A method according to claim 74, wherein POLY is a poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da.
 76. A method according to claim 74, wherein T is a bisphosphonate.
 77. A method according to claim 72, wherein the hydroxyapatite-targeting, biologically active polymeric structure has the following structure: T—L—POLY_(a)—R(—POLY_(b)—L′—A)_(q) wherein POLY_(a) and POLY_(b) are water-soluble and non-peptidic polymer backbones that may be the same or different; each A is independently selected from the group consisting of hydroxyapatite-targeting moieties and biologically active agents, with the proviso that at least one A is a biologically active agent; R is a central core molecule; L and L′ are linkers which may be the same or different; T is a hydroxyapatite-targeting moiety; and q is an integer from 2 to about 300; and wherein at least one of POLY_(a), POLY_(b), L and L′ comprises a hydrolytically or enzymatically degradable linkage.
 78. A method according to claim 77, wherein POLY_(a) and POLY_(b) are poly(ethylene glycol).
 79. A method according to claim 72, wherein said administering step comprises administering the polymeric structure by a route selected from the group consisting of oral, pulmonary, intravenous, subcutaneous, intramuscular, buccal, nasal, ocular, and rectal.
 80. A method according to claim 72, wherein said administering step comprises administering the polymeric structure with at least one pharmaceutically acceptable carrier, excipient or diluent. 